In performing surgery a surgeon employs surgical instruments that generally have a shaft and a tip. The tip may be, for example, a blade for cutting tissue of a patient, such as brain tissue. The shaft connects the tip to a portion that is held by the surgeon in the surgeon's hand and via which the surgeon can manipulate the instrument so that the tip cuts, or otherwise manipulates, contacts or is proximate to, tissue. The shaft of the instrument has thickness and the shaft thereby blocks the view of the surgeon of a portion of the patient's body tissues between the tip of the instrument and the portion of the instrument held by the patient.
It is often important for the surgeon to obtain measurements related to the operation in real-time during surgery, such as the patient's blood pressure and heart rate. In some systems a camera may track the instrument and calculate the distance between the tip of the instrument and the patient's tissue. Such information may be displayed to the surgeon on a computer display screen. However, this requires that the surgeon significantly change his focus to view the information and then change it again back to the tissue being operated on. Such changes are undesirable because they may be detrimental to optimal performance of delicate surgery, such as surgery performed on the brain. It would be preferable for the surgeon to not have to significantly change his or her focus repeatedly during the surgery.